1. Field of the Invention
The present invention relates to a power supply control device for a copier, printer, facsimile apparatus or similar image forming apparatus and more particularly to a power supply control device capable of obviating accidental error detection when the power switch of an image forming apparatus is turned off.
2. Description of the Background Art
Generally, when a power switch provided on an image forming apparatus is turned off, power supply to the entire system of the apparatus including controllers is shut off. Therefore, when the operator of the apparatus inadvertently turns off the power switch, a control unit including a CPU (Central Processing Unit) automatically stops executing control without recognizing the inadvertent turn-off of the power switch. This gives rise to the following problems.
When the power switch is turned off while, e.g., printing is under way, the entire drive system and entire control system of the apparatus stop operating. In this case, if the drive system operates in an unusual manner due to the drop of a power supply voltage before the control system stops operating, the control unit detects the unusual operation and writes it in, e.g., a nonvolatile memory. That is, an error history is written to the memory despite that the apparatus is free from an error. Further, in an advanced image forming apparatus with a server function implemented by a hard disk, when the power switch of the apparatus is turned off during writing of data in the hard disk or during access to the hard disk, it is likely that data stored in the hard disk are destroyed. Moreover, assume that the power switch is turned off while, e.g., a motor is in rotation, causing the control system stop operating due to voltage drop before the voltage drop of the drive system. Then, it is likely that mechanical arrangements are damaged due to unusual rotation of the motor.
To obviate the above occurrences, it is necessary for the control unit to execute adequate control when the operator turns off the power switch at an unexpected timing. For this purpose, Japanese Patent Laid-Open Publication No. 2000-172348, for example, proposes a power supply control scheme using a power supply control switch that does not directly shut off a commercial power supply. When the power supply control switch remains in a shutoff state for a preselected period of time, a power supply shutoff circuit shuts off power supply. More specifically, after the turn-off of a power switch has been detected, necessary control relating to shutoff is executed and then power supply is shut off. This kind of scheme, however, is apt to fail to shutoff power supply in the event of emergency, e.g., when a CPU runs away or when a heater included in a fixing unit malfunctions.